E4511ps4-txt - Problem 4 Consider a dislocation loop a...

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page 1 Engineering 45 The Structure and Properties of Materials Spring, 2011 Problem Set No. 4 Problem 1: (a) Would you expect the element P to be a donor or an acceptor defect in Si? (b) Suppose Si were substituted for As in the ß-ZnS compound GaAs. Would you expect the defect to be a donor, an acceptor or an electrically neutral defect? Problem 2: (a) Draw an edge dislocation in a simple cubic structure and define its Burgers' vector, b . (b) Describe how the dislocation can move by "glide" and "climb". (c) Show how the glide of an edge dislocation leads to plastic deformation. Problem 3: (a) Describe how you might create a screw dislocation in a crystal, and show how its Burgers’ vector, b , is related to its line length. (b) Show how the motion of a screw dislocation can lead to plastic deformation.
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Unformatted text preview: Problem 4: Consider a dislocation loop: a circular dislocation in a plane. The Burgers’ vector is the same everywhere inside the loop. Assume it lies in the plane of the loop. (a) Show that the dislocation is an edge dislocation at precisely two points on its circumference, and is a screw dislocation at precisely two other points. (b) Describe how the homogeneous expansion of the loop leads to plastic deformation. Problem 5: Cu and NaCl are both crystalline solids. An NaCl crystal has a strong tendency to take the form of a cube while a Cu crystal has little preference for a particular shape (in fact, given complete freedom of shape, it would tend to be nearly spherical). Explain this behavior in terms of the surface structure....
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